The body image is a mental representation of one's physical appearance, constructed by the brain from past experiences and present sensations. It is an essential component of self-identity, which, when altered, can have dramatic effects on how one perceives oneself. For example, a small proportion of migraine sufferers experience visual hallucinations just before the onset of a headache, in which the body parts appear larger or smaller than they actually are. Lewis Carroll, who is known to have suffered from migraines, documented such hallucinations in Alice in Wonderland.
These body image distortions can have bizarre consequences. Otherwise healthy people report that they have always percived a part of their body as feeling "wrong," and opt to have it removed by amputation; some brain-damaged or psychiatric patients experience alien hand syndrome, in which they deny ownership of a limb, and insist that it is under the control of external forces.
Our perception of our own body can, of course, be easily manipulated. As we walk through a house of mirrors at a fair, for example, we may view ourselves as being very short and fat one minute, and then very tall and thin the next. By looking through a pair of binoculars, a limb can be made to appear disproportionately large or small.
One might think that such temporary manipulations would be of little consequence. After all, we know that our body doesn’t really look like that. However, a study published recently in the journal Current Biology shows that a simple manipulation of the visual image of one's body can significantly alter the perception of pain. These findings have important implications for how clinicians manage the treatment of pain.
Binoculars as Pain Killers
For the study, Lorimer Moseley of Oxford University and his colleagues recruited 10 participants, all of whom suffer from chronic pain in their right arm. The participants were asked to perform a set of movements with their right arm, under different conditions. In one condition, they observed their limb through a pair of binoculars, which magnified their hand to twice its normal size; in another, the binoculars were inverted so that their hands appeared smaller than they actually were.
As they performed the arm movements, the participants were asked to rate the amount of pain they experienced. Each one reported that the pain they felt became markedly worse when they moved their limb. Surprisingly though, every participant also reported that the extent to which their pain increased depended on how their vision had been manipulated. They reported the greatest increase in pain when they saw a magnified view of their hand, and the smallest increase when their hands were minified.
The researchers used an objective measure to verify the participants' reports of the pain they experienced—they measured the swelling of the fingers induced by the movements, and compared it with the unaffected hand. Sure enough, there was more inflammation following the movements when the participants saw a magnified view of their hands. By contrast, the researchers measured less swelling of the fingers following the movements performed with a minified view, in line with the participants' subjective reports of experiencing less pain under that condition.
Furthermore, the visual manipulations also affected the time taken for the participants' pain to return to the level it was at before the movements were performed: recovery to the pre-movement level of pain was quickest in the "minified" condition and slowest in the "magnified" condition.
These findings clearly demonstrate that the mere appearance of a limb—making it appear larger or smaller than normal—can directly modulate our experience of bodily pain. Exactly how distorting the body image in this way affects the perception of pain remains unclear.
One possibility is that magnifying the view of the hand enhances the sense of touch, whereas minifying it has the opposite effect. Some evidence for this comes from a 2001 study, which showed that magnifying subjects' arms improved their ability to discriminate between needle points placed close to each other.
Alternatively, the authors suggest that manipulating the image of the hand may have altered the participants' sense of "ownership" of their painful limb. Viewing magnified images of their hands may have made the participants more aware that the limb belonged to them, thus increasing their sensitivity to the painful sensations originating from it. Conversely, the minifying condition may have "alienated" the participants' arms, reducing their sense of ownership of their limb and thus desensitizing them to the pain felt in them.
Regardless of the mechanism, these findings could lead to new methods for improving the rehabilitation regimes administered to patients with a wide variety of conditions. Most obviously, they point to a simple way for the effective management of chronic pain. They may also prove to be useful in helping patients with alien hand syndrome and related conditions to regain ownership of their limbs.
It is now well known that physical changes to the body alter the brain's representation of it. In the case of amputation, for example, the changes that take place in the brain lead to phenomena such as phantom pain, in which painful sensations are felt from the missing limb. This relationship between body and brain is referred to as "bottom-up"; this study, however, suggests a "top-down" relationship—it seems that the visual inputs (the participants' views of their hands) were sufficient to override the tactile sensations (the felt pain), by fooling the brain into modulating them.
Are you a scientist? Have you recently read a peer-reviewed paper that you want to write about? Then contact Mind Matters editor Jonah Lehrer, the science writer behind the blog The Frontal Cortex and the book Proust Was a Neuroscientist. His next book, How We Decide, will be available in February 2009.